Implementing Online User Counting with Redis Sorted Sets (ZSET)

1. Introduction

Online user statistics is a common requirement; this article shows how to implement it using Redis sorted sets (ZSET).

2. Core Redis Commands

The four essential commands are zadd , zrangeByScore , zremrangeByScore , and zrem .

3. Determining Whether a User Is Online

For authenticated sites, token validity is used; for public sites, a unique identifier can be generated via browser fingerprinting (e.g., FingerprintJS or ClientJS). Example JavaScript code is provided.

// Install: npm install @fingerprintjs/fingerprintjs
// Usage example:
import FingerprintJS from '@fingerprintjs/fingerprintjs';
FingerprintJS.load().then(fp => {
  fp.get().then(result => {
    const visitorId = result.visitorId;
    console.log(visitorId);
  });
});

4. Adding an Online User

Use zadd with the key (e.g., "user.active"), a score representing the expiration timestamp, and the user token as the member. Java example shown.

LocalDateTime expireTime = LocalDateTime.now().plusSeconds(expireTimeout);
String expireTimeStr = DateUtil.formatFullTime(expireTime);
redisService.zadd("user.active", Double.parseDouble(expireTimeStr), userToken);

5. Querying Online Users

Use zrangeByScore with a minimum of the current time and a maximum of +inf to retrieve all active members; the set size equals the online count.

String now = DateUtil.formatFullTime(LocalDateTime.now());
Set
userOnlineStringSet = redisService.zrangeByScore("user.active", now, "+inf");

6. Removing Expired Users

Schedule a task that calls zremrangeByScore with a range from -inf to the current time to delete stale entries.

String now = DateUtil.formatFullTime(LocalDateTime.now());
redisService.zremrangeByScore("user.active", "-inf", now);

7. Removing a User on Logout

When a user logs out, call zrem to delete the member from the set.

redisService.zrem("user.active", "xxx");

Conclusion

The approach creates a ZSET where the key is the online‑user set, members are user identifiers, and scores are expiration timestamps, enabling simple add, query, and cleanup operations.